The metabolism of nucleoside analogues which possess antiviral activity is currently being studied. The metabolism of three thymidine analogues which have been shown to be effective in the treatment of herpetic keratitis is the major focus of the study. Tritium labeled 5-trifluoromethyl-2'-deoxyuridine (F3TdR);5-iodo-5'-amino-2', 5'dideoxyuridine (AIU); and 5-iodo-2'-deoxyuridine (IDU), are used to treat noninfected control rabbits and rabbits with herpetic keratitis. After topical application of the radioactive antivirals, eyes are enucleated and serum collected. Eyes are dissected into the component structures and fractionated with 0.4N perchloric acid for analysis. The analyses include scintillation counting to determine the concentration per gram of tissue. Cold acid soluble and insoluble fractions are also analyzed by thin layer chromatography to identify the radioactive components. Upon identifcation of the metabolites, each is tested for cytotoxicity and antiviral activity in tissue culture. Cytotoxicity is measured by growth inhibition, viability and plating efficiency. Thymidine analogues were found to be distributed in the following manner: 12 hours after topical application - cornea, 1.36 pmoles/g wet weight; conjuctiva, 1.86 pmoles/g; iris, 1.13 pmoles/g; lens, 0.37 pmoles/g; serum 0.37 pmoles/0.1 ml; aqueous 0.29 pmoles/0.1 ml. The catabolites of F3TdR were 5 carboxyuracil (5-COOH U) and 5 carboxy 2'deoxyuridine (5-COOH UdR). The primary catabolite of IDU detected was iodouracil. No catabolites of AIU have been found. Cytotoxicity as measured by decreased plating efficiency indicated that F3TdR induced cytotoxicity at 10 to the minus 7th power M while 5-COOH-UdR was toxic at 10 to the minus 5th power M and 5 carboxyuracil showed no toxicity at 10 to the minus 4th power M. IDU showed toxicity at 10 to the minus 6th power M while IU showed toxicity at 10 to the minus 4th power M. AIU showed no toxicity at concentrations as high as 10 to the minus 4th power M, the highest concentration tested. None of the nucleoside antivirals nor their metabolites showed cytotoxicity to corneal endothelial cells by pefusion under a specular microscope. Future studies will pursue the distribution of anabolic products in ocular tissues and mechanisms of cytotoxicity.